This invention relates to a protection circuit for protecting a transistor, etc., for controlling the conduction state of an electric current to an inductive load and in particular to a protection circuit applied to power supply control in various vehicle-installed power supply distribution sections.
FIG. 2 is a circuit diagram of a protection circuit in a related art and a circuit configuration incorporating the protection circuit. In this kind of protection circuit, a diode 3 is connected in parallel with an inductive load (for example, a motor) 1 as a measure against surge voltage occurring when the load 1 is powered off, as shown in FIG. 2. In the figure, numeral 5 denotes a MOSFET (transistor) for controlling the energization state of the load 1 and numeral 7 denotes a drive circuit for driving the MOSFET 5. Power supply current supplied from a power supply line 9 is supplied via the MOSFET 5 to the load 1.
However, in the protection circuit in the related art, if the circuit configuration shown in FIG. 2 is connected to the power supply line 9 in incorrect polarity by mistake, the power supply current from the power supply line 9 (reverse current) flows into the MOSFET 5 via the diode 3 rather than via the load 1 as on a route A shown in FIG. 2 and there is a fear of breaking the MOSFET 5.
It is therefore an object of the invention to provide a protection circuit capable of preventing an overcurrent from flowing into a transistor if a circuit configuration incorporating the invention is connected to a power supply in incorrect polarity by mistake while taking a measure against surge voltage occurring when a load in the circuit configuration is powered off.
To the end, according to the invention, there is provided a protection circuit being provided for a circuit configuration comprising an inductive load and a transistor for controlling the energization state of the load, the protection circuit comprising a diode being connected in parallel to the load and having a forward direction set reverse to the conduction direction of a power supply current to the load; and a Zener diode being placed in a conducting path between one terminal of the diode and one terminal of the load corresponding to the terminal of the diode and having a forward direction matched with the conduction direction of the power supply current.
Preferably, the transistor may be placed upstream in the conduction direction of the power supply current relative to the load.